ensure_pango_color: Support hex values with alpha

Reference: https://github.com/awesomeWM/awesome/pull/2129#discussion_r155397507
Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/gears/color.lua

@@ -338,6 +338,11 @@ end
 -- @treturn string color if it is valid, else fallback.
 function color.ensure_pango_color(check_color, fallback)
     check_color = tostring(check_color)
+    -- Pango markup supports alpha, PangoColor does not. Thus, check for this.
+    local len = #check_color
+    if string.match(check_color, "^#%x+$") and (len == 5 or len == 9 or len == 17) then
+        return check_color
+    end
     return Pango.Color.parse(Pango.Color(), check_color) and check_color or fallback or "black"
 end
 

spec/gears/color_spec.lua

@@ -271,7 +271,7 @@ describe("gears.color", function()
     describe("ensure_pango_color", function()
         -- Successful cases
         for _, value in ipairs{ "red", "cyan", "black", "#f00", "#014578",
-                    "#01ef01ef01ef"
+                    "#01ef01ef01ef", "#f00f", "#014578ab", "#01ef01ef01ef01ef"
                 } do
             it(value, function()
                 assert.is.same(value, color.ensure_pango_color(value))
@@ -282,6 +282,11 @@ describe("gears.color", function()
             assert.is.same("black", color.ensure_pango_color("#abz"))
         end)
 
+        it("#fedcba98765432", function()
+            -- Only one, two or four characters per channel are supported
+            assert.is.same("black", color.ensure_pango_color("#fedcba98765432"))
+        end)
+
         it("fallback", function()
             assert.is.same("zzz", color.ensure_pango_color("#abz", "zzz"))
         end)